Define the following terms: crystalline solid, lattice point, unit cell, coordination number, closest packing.

Imagine a grid that extends infinitely in three dimensions, much like a 3D chessboard where atoms or groups of atoms stake their claim on certain positions. In crystallography, these positions are known as lattice points. Each lattice point marks a location where there is a repeating or symmetrical arrangement of particles within the crystalline structure.

These points are fundamental in defining the structure of solids since they represent environments that are identical throughout the crystal. Whenever you see a diagram of a crystal, think of the lattice points as pins on a map that consistently mark every area where an atom should be.

To comprehensively understand the architecture of crystals, imagine a single brick that, when stacked together with identical bricks, constructs an entire building. This brick is akin to the unit cell in a crystalline solid. The unit cell is essentially the smallest portion of the crystal lattice that still retains the overall symmetry and structure of the entire crystal.

Depending on the crystal's symmetry and atomic arrangement, the shape of the unit cell can vary, with common types being cubic, tetragonal, orthorhombic, among others. By replicating the unit cell in three dimensions — up and down, left and right, back and forth — the entire crystal lattice is formed. The unit cell is crucial for understanding the physical properties of the crystal, including its density and how it interacts with light.

A term frequently used when talking about chemical bonds and crystalline structures is the coordination number. This number indicates the number of immediate neighboring atoms to a particular atom within the structure. Think of it as the measure of how many friends an atom has in its closest circle in the crystal party.

The coordination number plays a significant role in defining the stability and strength of a crystalline solid, as well as the dimensions of its unit cell. It's determined by the spatial arrangement of atoms around the central atom and can vary greatly among different crystals. For example, in a simple cubic structure, an atom's coordination number is 6, which means each atom is surrounded by 6 immediate neighbors.

To visualize closest packing, think about how oranges are stacked at a grocery store; you'll notice that the grocer arranges the oranges so that they take up the least amount of space and are snugly fit together. This is very similar to how atoms are arranged in the most efficient manner within a crystal.

The concept aims to achieve the highest packing density, the percentage of space that is filled by the atoms, which for closest packing is about 74%. There are several types of closest packing, such as face-centered cubic (FCC) and hexagonal close-packed (HCP), each with a different arrangement of spheres (atoms) and pathways through which they are stacked. This arrangement directly influences many properties of the material, including its density, melting point, and how it deforms under stress.